1. Field of the Invention
This invention relates to a control and supervisory signal transmission system, and in particular to a control and supervisory signal transmission system in which a parallel control signal from a controller is converted into a serial signal, the serial signal is transmitted to a remote device and serial-parallel converted in a controlled device side to drive the device, a supervisory signal from a sensor section for detecting the status of the device is serial-parallel converted and provided to the controller, and the control signal is superimposed on a clock signal on which power-supply signal is superimposed, and in addition, the supervisory signal is superimposed on these signals.
2. Description of the Related Art
In the field of automatic control, a technology is widely used in which control signals are transmitted from a controller such as a sequence controller, a programmable controller, or a computer, to a large number of remote controlled devices (for example, motors, solenoids, solenoid valves, relays, thyristors, or the like) for driving and controlling them and supervisory signals are transmitted from sensors that detect states of devices (on/off state of reed switches, micro-switches, push-button switches) to the controller.
There are problems associated with the above-described technology: because a number of lines, including power lines, control-signal lines, and ground lines, are used to connect a controller with controlled devices and the controller with sensors, wiring becomes increasingly difficult to provide in high-density component layouts and wiring space is reduced and becomes costly in concert with the miniaturization of today's controlled devices.
There have been proposed two approaches: “Method for signal serial-parallel conversion” (Japanese Patent Application Serial No. Sho62(1987)–229978) and “Serial transmission system for parallel sensor signals” (Japanese Patent Application Serial No. Sho62(1987)-247245): According to the approaches, one (1-bit) control signal (or a sensor signal) can be superimposed on each clock on a clock signal line which includes power supply, thereby allowing implementation of a transmission system in which wiring between a controller and controlled devices and between the controller and sensors is reduced.
According to another approach, “Control and supervisory signal transmission method” (Japanese Patent Application Serial No. Hei1(1989)-140826), input units and output units are connected to a parent station and the parent station outputs a clock signal which is superimposed on a power supply signal onto a common data signal line, thereby implementing high-speed bi-directional signal transmission between a controller and controlled devices and the controller and sensors in a simple configuration. That is, a transmission system can be configured with a smaller number of lines, the costs of wiring can be reduced, the connection arrangement of the units can be simplified, and addresses can be assigned to the units flexibly and therefore a unit can be deleted or added to any position.
According to the prior-art arrangements described above, high-speed bi-directional signal transmission can be provided between a controller and controlled devices and between the controller and sensors. However, a signal provided from the controller to a controlled device (hereinafter called a control signal) and a signal provided from a sensor to the controller (hereinafter called a supervisory signal) are outputted onto a common data signal line. Accordingly, both of the signals cannot simultaneously be transmitted. That is, a control signal and a supervisory signal cannot be transmitted at the same time. Instead, they can be transmitted mutually exclusively. Therefore, a time period during which the control signal is transmitted and a separate time period during which the supervisory signal is transmitted must be provided for the common data signal line.
Furthermore, although power supply is superimposed on a clock signal transmitted over a common data signal line, the line transmits for the most part the clock. Accordingly, average power that can be transmitted over the common data line is limited. If transmittable average power could be increased as much as possible, two (24V and 0V) power supply lines would be able to be eliminated and the length of two (D+ and D−) common data signal lines would be able to be increased, thereby allowing a control signal to be transmitted to and a supervisory signal to be obtained from a place in a controlled device where available wiring space is small.